1. Field of the Invention
This invention relates to a silver halide photographic emulsion and, more particularly, it is concerned with a silver halide photographic emulsion for direct positives containing a nitro-substitued fluorene derivative.
2. Description of the Prior Art
When a silver halide light-sensitive material is exposed to a light within the light-sensitive wavelength range of this light-sensitive material and developed, the blackened density increases and leads to a maximum value with an increase of the quantity of exposure, but decreases again and forms finally a positive image with a further increase in the quantity of exposure. Such a phenomenon is generally called "solarization". A similar reversal development to that by light is found also with a silver halide which has been fogged optically or chemically in the process of producing the silver halide emulsion. In this specification, the term "direct positive emulsion" means an emulsion which is processed to form a positive image by ordinary exposure and development.
The instant invention relates to a direct positive emulsion sensitive to the blue color region due to the incorporation into the emulsion of an organic desensitizer. For example, in U.S. Pat. No. 3,501,307 an emulsion of the type that is reversed by the use of an organic desensitizer, such as pinakryptol yellow or 5-m-nitrobenzilidene rhodanine, is disclosed. It is known that an emulsion fogged using a relatively small amount of a gold compound or salt and a reducing agent can be advantageously used to obtain a particularly high reversal sensitivity and an organic desensitizer is effective for the sensitization of such an emulsion. However, known organic desensitizers which have hitherto been used for the sensitization in the blue color region have various disadvantages.
In general, an organic desensitizer having a relatively high reversal sensitivity reduces the maximum density of an original emulsion, while, on the other hand, a desensitizer which does not reduce the maximum density gives only a low reversal sensitivity. In this specification, an organic desensitizer-free emulsion will hereinafter be referred to by the term "original emulsion". For example, pinakryptol yellow reduces the maximum density of an original emulsion and colors a photographic material used yellow. 3-ethyl-5-m-nitrobenzilidene rhodanine does not reduce the maximum density as much, but the clarity is bad, that is, the value of minimum density is large. 5-m-Nitrobenzilidene rhodanine does not reduce the maximum density as much and the value of the minimum density is small, that is, the clarity is good, but a low reversal sensitivity is obtained. Therefore, it is very important to find an organic desensitizer for sensitizing in the blue color region, which is capable of giving a high reversal sensitivity without reducing the maximum density and yet retaining a small minimum density.
It is a principal object of the invention to provide a silver halide photographic emulsion for direct positives, which contains an organic desensitizer for sensitization in the blue color region, capable of giving a high sensitivity, being substantially free from any residual coloring and without reducing the maximum density of an original emulsion.